It seems to me, from this thread, that a nice compromise, considering a “purchase” option is not available, not ideal or not the focus of the experiment in case it still is an experiment, that tracking that does not gathers individual data and just evaluates general response, would be effective and privacy respecting.
Illustrating the idea: Place a “Click here to see more of X” in an app, then seeing how many clicks it get, then testing three month with “Click here to see more of Y” and comparing click results… I cannot figure out who anyone is, but I can tell an ad’s effectiveness.
About relevancy, I get that it would be only acceptable if super relevant to where it shows, like referencing other calculator apps in a calculator app or other books of the same subject in the end of a book.
About intrusiveness… where exactly to place the ad? Cannot be buried behind three screens and a scroll, else nobody will ever see, but also cannot be those infuriating interruptions else one will throw the phone in a wall…